Piezoelectric sensors are well known. They are used for sensing material properties such as viscosity and density, for detecting the presence of certain materials in an environment, for measuring purity of fluid substance, and the like. Structures known for acoustic sensing range from the simple crystal resonator, crystal filters, acoustic plate mode devices, Lamb wave devices, and the like. Briefly, these devices comprise a substrate of piezoelectric material such as quartz, langasite or lithium niobate, or thin films of piezoelectric material, such as aluminum nitride, zinc oxide, or cadmium sulfide, on a non-piezoelectric substrate. The substrate has at least one active piezoelectric surface area, which in most cases is highly polished. Formed on the surface are input and output transducers for the purpose of converting input electrical energy to acoustic energy within the substrate and reconverting the acoustic energy to an electric output signal. These transducers may consist of parallel plate and co-planar plate (bulk-generated acoustic wave) or periodic interdigitated (surface-generated acoustic wave) transducers. It is noted that a single transducer may act both as the input and the output transducer.
Piezoelectric materials interconvert electrical and mechanical signals and energy, allowing an electrical circuit to be responsive to a physical effect on the mechanical properties of a vibrating system. The literature presents countless instances of detecting temperature, pressure, added mass, viscoelastic variations, magnetic fields and the like using these sensors. The interactions between the devices and the electronic circuits have historically included the response of the device's phase or amplitude at a given frequency and changes in the resonant frequency or damping of a natural resonant mode of the device. Both phase delay and resonant frequency can be employed to create an oscillator circuit, ultimately providing frequency change as the circuit response to ambient physical influences.
Piezoelectric sensors can be designed to operate while being fully immersed in fluid. However the sensitive electronics are then subjected to, in the least, noise signals and reading errors and, in the extreme, to corrosion or even explosive hazards. Passivation of the electronics surface is well known and is suitable in some limited applications, as seen for the Love Wave and surface transverse wave (STW) sensors. However passivation is not complete and electrical components of the circuit are still exposed to capacitive loading and noise injection. Moreover, most passivation methods require the use of material having poor acoustic characteristics compared to single crystal materials. Finally, these passivated surface wave based sensors exhibit undesirably high shear rate for many liquid phase measurements. While such sensors potentially address many sensor applications, they are not ideal, for instance, in measuring fluids in oil production, especially in down-well environments.
More preferably the frequency of measurement is maintained below approximately 10 MHz and the preferred geometries employ the thickness of the piezoelectric plate to form a waveguide.
In most applications only the surface opposite the transducers is in direct or indirect contact with the fluid being measured and interfaces acoustic energy to and from it. In addition to the interface function, the piezoelectric material forms a protective membrane between the fluid and a cavity containing electrical components of the sensor.
As the cavity behind the piezoelectric plate material is commonly not pressurized to the same level of the fluid, the piezoelectric plate acts as a membrane between the high and low pressure environments, and is exposed to the pressure difference between the fluid and the pressure within the cavity. Therefore, the finite strength of the material limits the operating pressure to which the sensor may be exposed. Even if the material is sufficiently strong to withstand the pressure, the nonlinear effect on the sensor of membrane flexure will severely affect the sensor characteristics.
As increasing material thickness increases the membrane strength, a simple solution will be to increase the thickness of the piezoelectric membrane. This however suffers from reduced acoustic coupling, reduced efficiency and dynamic range, and other disadvantages, whether the piezoelectric material is operated at fundamental or at overtone operating mode.
Yet another disadvantage of the present piezoelectric sensors is lack of resistance to harsh chemicals, abrasion, and the like. Thus for example, an excellent piezoelectric material such as langasite will deteriorate in certain acidic environments and other piezoelectric materials such as lithium tetraborate are water soluble.
In PCT application No. PCT/US06/15510 to Andle, I disclosed a composite acoustic wave device (AWD) which is adapted for operation at high ambient pressures. The AWD comprises two piezoelectric plates in a symmetric back to back relationship, with electrodes disposed between the plates. The plates are bonded so as to neutralize the effects of external pressure, which is isobaric under immersion. This application is incorporated herein by reference in its entirety.
Several devices are reported in the literature, such as “Measurement of the equivalent circuit parameters of chemical interface layers on bulk acoustic wave resonator” by G J Gouws, R. C. Holt, and J Zhen, Proceedings of the 2004 IEEE International Frequency Control Symposium and exposition, “PMMA polymer film characterization using thickness-shear mode (TSM) quartz resonator” by Boima Morray, Suiqiong Ii, Jeanne Hossenlopp, Richard Cernosek, and Fabien Josse, 2002 IEEE International Frequency Control Symposium and exposition, and others. Those devices add a layer of polymer, or metal deposited by thin or thick film technologies to the exposed sensing surface. In some cases such layers may be a quarter or even half wavelength thick. In such devices, the lateral extent of the added layer is limited by the piezoelectric plate size, and does not support the piezoelectric, but is rather supported therefrom. Thus film deposition methods do not provide additional resistance to pressure or encapsulation from environmental damage.
A high overtone bulk acoustic resonator, also known as HBAR, is a compressional wave device, comprising a piezoelectric layer grown on the end of a sapphire or garnet rod of a large number (at least over 100) of half wavelengths in length. An intentional acoustic mismatch between the sapphire and the piezoelectric plate allows the device to have a very high reflection of the energy trapped therein, and thus generate a number of extremely sharp transfer peaks. The sharp transfer peaks allow the HBAR to act as an extremely high Q filter. However, the weak acoustic coupling and the compressional wave operation mode make the device ill suited for liquid-phase sensor duty, which requires a shear wave. Furthermore to obtain a highly responsive electrical response to a mechanical effect requires as tight coupling as possible to work efficiently. Furthermore, as will be seen infra, unlike the HBAR, the present invention attempts to minimize wave reflection at the energy interface, while the HBAR attempts to maximize such reflection in order to generate the high Q, multiple mode operation. The present invention is generally directed to a narrow band, frequency selective, and resonant finite impulse device, while the HBAR is directed to a compressional wave, multiple frequency device.
Many technology areas may benefit from measuring fluid with low sensitivity to pressure variations or at high pressure levels, as well as providing for harsh chemical or abrasive materials. Examples of such technologies include by way of a non-limiting example, gas production, oil well and oil pipes, hydraulic systems, in-service lubricant monitoring, injection molding equipment, anti terror detection system for detection of biological and chemical substances, and the like. Therefore there is a long felt and heretofore unanswered need in the industry for an electro acoustic sensor, which in various embodiments is capable of operating with low sensitivity to pressure variations, in high ambient pressure environments, and/or in harsh chemical or abrasive environments. The present invention aims to provide a solution to any single one of the above conditions, or to any combination thereof.